XXVTII. On a New Correction in the Construction of the Double 

 Achromatic Ohject-gla,i.i. By Richard Pottek, Esq. B.A. 

 Queen,?' College. 



[Read April 30, 1838.] 



The acliromatisni of the compound object-glass of telescopes has 

 never, that I am aware of, been investigated otherwise than for very 

 small pencils. 



Sir John Herschel, in his elaborate and excellent paper on the 

 aberrations of compound lenses and object-glasses, published in the 

 Transactions of the Royal Society for 1821, when noticing the investi- 

 gations of Clairaut, Euler, and D'Alembert, makes no mention of any 

 higher approximation having been attempted; and he himself follows 

 no other method for the chromatic dispersion, although he has pursued 

 the subject of the spherical aberration so far as to render the object- 

 glass free from it, for astronomical and terrestrial objects at the same 

 time. Indeed, from the following passage in the same paper, it is 

 clear that he did not suspect the existence of any unconsidered residual 

 dispersion, of the magnitude of that which I am about to discuss; for 

 he says, "The simplest considerations, indeed, suffice for the correction 

 of that part of the aberration which arises from the different refrangi- 

 bility of the differently coloured rays ; and accordingly this part of the 

 mathematical theory of refracting telescopes was soon brought to per- 

 fection, and has received no important accession since the original in- 

 vention of the achromatic object-glass." 



